Heterostructure Phototransistor Arrays for Free-Space Laser Communications

We explore using heterostructure phototransistors (HPT) to reduce the complexity and, hence, the size, weight, and power (SWaP) of a space borne laser communications system, by combining pointing/acquisition/tracking (PAT) and wideband communications functions in a single sensor assembly. Benefits of this approach include a reduced number of sensor elements, fewer receive optical paths requiring alignment over life, and subsequent SWaP improvements in interface electronics. Because of the uniform gain without excess noise, HPT arrays are expected to have superior performance compared to PIN and APD photodiodes for receive sensitivity or signal-to-noise ratio required for communication and/or PAT functions. The HPT also offers the potential to remove the point-ahead mirror, if the sensor is used in a look-behind mode, and, in the ultimate scenario, the sensor would be sampled at the data rate, if the FOV is sufficient to encompass the entire jitter spectrum, thus, negating the need for a fast steering mirror (FSM) for beam stabilization.